Parker-Pope references the so-called "love-rat gene," which helps regulate the hormone vasopressin and which, in one variation, can predispose men toward unstable relationships. But in a reminder that biology isn't destiny, she also describes a McGill University study in which none were "train" to resist the temptation to cheat. The men, all in committed relationships, were asked to imagine encountering an attractive woman while their girlfriends were away. Researchers then asked some of them to complete the sentence, "When she approaches me, I will __________ to protect my relationship." Surprisingly, this simple task influenced the men's behavior. When they later played a video game that included rooms with subliminal images of women, those who had completed the sentence went to these rooms far less often than those who had not.

Of course, subconsciously looking at women in a video game isn't the same as cheating, and researchers haven't yet studied how their sentence-completion task might affect relationships in the real world. But when people so often use science to argue that men have a natural and unavoidable imperative to spread their seed, it's interesting to read a study that suggests the opposite — that humans can regulate their own behavior in relatively simple ways. Parker-Pope also describes another study that sheds light on what keeps couples together long-term. Psychologist Arthur Aron thinks the key is the quality called "self-expansion," which he assesses with questions like the following:

How much does your partner provide a source of exciting experiences? How much has knowing your partner made you a better person? How much do you see your partner as a way to expand your own capabilities?

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Aron and his team have even developed tasks that stimulate self-expansion — difficult activities that couples complete together. They've found that the succeeding at such activities increases couples' love and satisfaction with their relationships. Again, it's not possible to fully study love in a lab, and what makes any relationships succeed or fail is probably a complex combination of factors. But Aron's theories are interesting because they base commitment not on a feeling of completion or having found the perfect person, but on an ability to grow together. In a culture where, at least for women, marriage is frequently treated as an endpoint, this emphasis on growth is powerful. Too often, we're told to focus all our energies on finding the right partner (or settling for the sort-of-right one), with the implication that we can then relax into a life of unchanging and unchallenging bliss. But if Aron and the McGill researchers are right, happy relationships are dynamic entities, and the much-vaunted quest for marriage may matter less than what people do when they get there.